Gold(I)-catalyzed intermolecular dioxolane addition to alkynes: the role of water

Density functional theory (DFT) calculations have been performed on a homogeneous gold-catalyzed reaction between acetylene/propyne and the cyclic ketal 2,2-dimethyl-1,3-dioxolane, DMDO, in the presence of water, with the aim of understanding the actual role played by water. After the addition of DMDO to the alkyne, hydrolysis may happen through two possible reaction routes. In the so-called H-route (previously proposed for similar intramolecular reactions), a water proton is initially added to the alkyne C atom still linked to gold and, afterwards, an OH group adds to a DMDO C atom to allow the release of acetone, whereas in the newly proposed OH-route, a water OH group firstly adds to the most substituted DMDO C atom with simultaneous addition of H to the alkyne C atom linked to gold. A 1,3-H transposition from the just added OH group allows the release of acetone. An intramolecular nucleophilic OH addition to the gold-activated alkene intermediate formed from both hydrolysis paths drives the system to the corresponding 1,3-dioxolane product. The H-route is unable to explain the formation of the dioxolane addition products (observed in similar intramolecular reactions) instead of those coming from the direct addition of water to the alkyne, since it is energetically more demanding than the direct hydration. However, OH-route goes through structures that are more stable than those in the water addition, so, it is the one actually happening for the reaction under study. The regioselective addition of DMDO to the internal C atom of propyne is predicted on the basis of the large polarity of the structures formed in this approach, which makes them capable of strong interactions with water.